The phosphoinositide 3-kinases (PI3Ks) are lipid and protein kinases involved in intracellular signal transduction. They act primarily through phosphorylation of phosphoinositides at the D3 position of the inositol ring, and are typically grouped into three classes (I, II, and III) based on their structure, function, and substrate specificity. The class I PI3Ks, denoted PI3Kα, PI3Kβ, PI3Kγ, and PI3Kδ, catalyze the phosphorylation of phosphatidylinositol-4,5-bisphosphate to phosphatidylinositol-3,4,5-trisphosphate, which functions as a second messenger whose binding to proteins containing pleckstrin homology domains, such as AKT, PDK1, Btk, GTPase activating proteins, and guanine nucleotide exchange factors, triggers a cascade of cellular processes involved with cell growth, survival, proliferation, apoptosis, adhesion, and migration, among others. See L. C. Cantley, Science 296:1655-57 (2002). Class I PI3K isoforms exist as heterodimers composed of a catalytic subunit, p110, and an associated regulatory subunit that controls their expression, activation, and subcellular localization. PI3Kα, PI3Kβ, and PI3Kδ associate with a regulatory subunit, p85, and are activated by growth factors and cytokines through a tyrosine kinase-dependent mechanism; PI3Kγ associates with two regulatory subunits, p101 and p84, and is activated by G-protein-coupled receptors. See C. Jimenez, et al., J. Biol. Chem., 277(44):41556-62 (2002) and C. Brock, et al., J. Cell. Biol., 160(1):89-99 (2003).
Although PI3Kα and PI3Kβ are expressed in many tissue types, PI3Kγ and PI3Kδ are predominantly expressed in leukocytes and are therefore thought to be attractive targets for treating inflammatory disorders and other diseases related to the immune system. See B. Vanhaesebroeck, et al., Trends Biochem. Sci. 30:194-204 (2005), C. Rommel et al., Nature Rev. Immunology, 7:191-201 (2007), and A. Ghigo et al., BioEssays 32:185-196 (2010). Recent preclinical studies support this view. For example, treatments with selective PI3Kγ inhibitors suppress the progression of joint inflammation and damage in mouse models of rheumatoid arthritis (RA), and reduce glomerulonephritis and extend survival in the MRL-lpr mouse model of systemic lupus erythematosus (SLE). See M. Camps et al., Nature Med. 11:936-43 (2005), G. S. Firestein, N. Engl. J. Med. 354:80-82 (2006), and S. Hayer et al., FASEB J 23:4288-98 (2009) (RA); see also D. F. Barber et al., Nature Med. 11:933-35 (2005) (SLE). A selective PI3Kγ inhibitor has also been shown to reduce formation and size of lesions in mouse models of early- and advanced-stage atherosclerosis, and to stabilize plaque formation thereby minimizing risks of plaque rupture and subsequent thrombosis and myocardial infarction. See A. Fougerat et al., Circulation 117:1310-17.2008. Treatments with PI3Kδ-selective inhibitors significantly reduce inflammation and associated bone and cartilage erosion following injection of wild type mice with an arthritogenic serum, attenuate allergic airway inflammation and hyper-responsiveness in a mouse model of asthma, and protect mice against anaphylactic allergic responses. See T. M. Randis et al., Eur. J. Immunol. 38:1215-24 (2008) (RA); K. S. Lee et al., FASEB J. 20:455-65 (2006) and H. S. Farghaly et al., Mol. Pharmacol. 73:1530-37 (2008) (asthma); K. Ali et al., Nature 431:1007-11 (2004) (anaphylaxis). Administration of a PI3Kγ and PI3Kδ dual selective inhibitor has been shown to be efficacious in murine models of allergic asthma and chronic obstructive pulmonary disease (COPD) and is cardioprotective in murine and porcine models of myocardial infraction (MI). See J. Doukas et al., J. Pharmacol. Exp. Ther. 328:758-65 (2009) (asthma and COPD); J. Doukas et al., Proc. Nat'l Acad. Sci. USA 103:19866-71 (2006) (MI).
Studies also suggest targeting one or more of the four class I PI3K isoforms may yield useful treatments for cancer. The gene encoding p110α is mutated frequently in common cancers, including breast, brain, prostate, colon, gastric, lung, and endometrial cancers. See Y. Samuels et al., Science 304:554 (2004) and Y. Samuels & K. Ericson, Curr. Opin. Oncol. 18(1):77-82 (2006). One of three amino acid substitutions in the helical or kinase domains of the enzyme are responsible for 80 percent of these mutations, which lead to significant upregulation of kinase activity and result in oncogenic transformation in cell culture and in animal models. See S. Kang et al., Proc. Nat'l Acad. Sci. USA 102(3):802-7 (2005) and A. Bader et al., Proc. Nat'l Acad. Sci. USA 103(5):1475-79 (2006). No such mutations have been identified in the other PI3K isoforms, though there is evidence they can contribute to the development and progression of malignancies. PI3Kδ is consistently overexpressed in acute myeloblastic leukemia and inhibitors of PI3Kδ can prevent the growth of leukemic cells. See P. Sujobert et al., Blood 106(3):1063-66 (2005); C. Billottet et al., Oncogene 25(50):6648-59 (2006). PI3Kγ expression is elevated in chronic myeloid leukemia. See F. Hickey & T. Cotter, J. Biol. Chem. 281(5):2441-50 (2006). Alterations in expression of PI3Kβ, PI3Kγ, and PI3Kδ have also been observed in cancers of the brain, colon and bladder. See C. Benistant et al., Oncogene, 19(44):5083-90 (2000), M. Mizoguchi et al., Brain Pathology 14(4):372-77 (2004), and C. Knobbe et al, Neuropathology Appl. Neurobiology 31(5):486-90 (2005). Moreover, all of these isoforms have been shown to be oncogenic in cell culture. See S. Kang et al. (2006).
Certain inhibitors of PI3K are described in WO2006/005915 and WO 2008/023180.